1. Field of the Invention
The present invention relates to the transmission of data within digital systems such as local area networks (LANs). More specifically, this invention relates to the predistortion of binary encoded data in such systems.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
The interest in local-area networks is steadily increasing. Local area networks facilitate economical data communication between computing systems clustered in a locality. Networks used widely in the art are known as token ring LAN's and Ethernet LAN's.
A token ring LAN is a circular network having a plurality of stations (nodes) interconnected in a ring topology. Access to the network is controlled by the possession of a signal "token". The token is a packet of signals that is passed from node to node. The node that has the token has control of the network with respect to the transmission of data to other nodes and the receipt of data from the same. When the node has completed a transmission, the token is released for acquisition by another node.
Ease of clock extraction has been an important factor bearing on the selection of a transmission code within LANs. In this regard the Manchester coding scheme is favored as allowing for relatively simple clock extraction, since this code produces a zero crossing at every midbit interval irrespective of the specific data pattern. In addition, the zero DC component carried by Manchester coded signals facilitates AC coupling by using transformers to provide protection.
In LAN systems employing a Manchester coding scheme the quality of the transmission is primarily determined by signal-to-noise ratio and the phase distortion introduced by the transmission line. Numerous techniques are known in the art for increasing signal-to-noise ratio, but detection of a signal degraded by transmission-induced phase distortion is generally more difficult. Specifically, a valid Manchester data waveform may be characterized as a sequence of short and long pulses of duration T/2 and T, respectively, where T signifies the reciprocal of the bit rate. As the spectral composition of the two pulse types are not identical, the propagation delay is different for each since conventional transmission lines exhibit a "low-pass" type transfer function. Accordingly, the spacing between transitions in the Manchester waveform will vary as a function of the data pattern. This phase distortion induced by the transmission line makes recovery of the clock frequency from the Manchester data more difficult and susceptible to error.
In order to counteract the effects of phase distortion over lengthy transmission lines it is known to provide a pre-equalizer at the transmitting end of the line, or to include a post-equalizer at the receiving end of the line. The equalizer serves to introduce a compensatory distortion opposite to that engendered by the tranmission line. However, since such distortion varies with line length the equalizer or filter must be adjusted for a particular transmission distance. This leads to system inflexibility since changes in line length require corresponding adjustment of the equalizer or filter.
An alternative technique for reducing phase distortion induced by transmission lines has been proposed by Widmer in U.S. Pat. No. 3,980,826, issued Sep. 14, 1976, and entitled "Means of Predistorting Digital Signals". Widmer discloses a method and means for transmitting a waveform of mixed frequency content over a transmission line in a manner designed to reduce distortion. The specific application addressed by Widmer relates to a bifrequency or biphase encoded binary data transmission system. Distortion is ostensibly minimized by reducing the low frequency content of the signal. This is accomplished by first distinguishing bits encoded at the lower frequency of the bifrequency encoded data, and by then making a step reduction in the amplitude of the low frequency bits. This process reduces the initial amplitude of the low frequency bits during the latter portion of each of the bit periods.
Nonetheless, the predistorted waveforms proposed by Widmer include signal harmonics having appreciable power density. The harmonics tend to radiate from commonly employed transmission media such as unshielded twisted pair cable. This parasitic radiation may engender crosstalk between system components or lead to errors in data transmission. Moreover, regulations promulgated by the Federal Communications Commission (FCC) limit the magnitude of signal harmonics which may be radiated by communications equipment and the like.
Accordingly, a need exists in the art for a technique for predistorting an encoded digital signal so as to minimize phase distortion and harmonic radiation during signal transmission.